Spotlight / 2003

Farmers who switched from sprinkler irrigation to drip systems (above) have cut their water use by 30-60%

Improving irrigation technology

Affordable innovations, tailored to local conditions, are readily adopted by resource-poor farmers...

Irrigated agriculture has driven much of the increase in global food production over recent decades. While only 20% of the world's farmland is irrigated, it produces 40% of our food supply. The highest yields obtained from irrigation are more than double the highest yields from rainfed agriculture - even low-input irrigation is more productive than high-input rainfed farming.

Why then is irrigated agriculture unlikely to expand as fast it has over the past? One consideration is cost: irrigation has been described as "one of the most subsidized activities in the world", and some studies have cast doubts on the economic returns on investment in large-scale irrigation schemes. The environmental costs of conventional irrigation are also high. High-intensity schemes are often blamed for waterlogging and soil salinization, which now affect 30% of irrigated land. Salinization is reducing the existing area under irrigation by up to 2% a year. To increase irrigation's contribution to food production, FAO says, what is needed is improved efficiency in the use of irrigation water.

Drip, drip, drip... If incentives are in place, farmers will adopt water-saving irrigation technologies. The main technologies likely to be used in developing countries, where labour is normally abundant but capital scarce, are underground and drip irrigation. Both technologies depend on the frequent application of small amounts of water as directly as possible to the roots of crops. A major advantage of water-saving technologies, particularly drip irrigation, is that as well as saving water they can increase yields and reduce the rate of salinization. Furthermore, since neither system brings water into contact with foliage, they can be used with brackish water for crops that are not too sensitive to salinity.

Some underground irrigation systems do not require costly equipment but are labour intensive. Indeed, one of the oldest methods of irrigation is placing porous clay jars in the soil around fruit trees and along crop rows. Porous or perforated pipes buried underground serve the same purpose, and can usually be used to irrigate two rows of crops, one on either side of the pipe. The rate of application cannot be controlled (although the frequency can) since it depends on the size of the perforations and soil characteristics.

Drip irrigation - a pressurized system that forces water through perforated pipes running above ground - has been applied to only a small part of the area for which it is suited. Though the technology is relatively simple, it does require both investment and maintenance - water emitters can easily become clogged. However, results from many countries show that farmers who switched from sprinkler irrigation to drip systems have cut their water use by 30-60%. Because plants are effectively 'spoon-fed' the optimal amount of water (and often fertilizer) when they need it, crop yields often increase at the same time.

Waste-water
Reducing pollution in the water used by farms, industries and urban areas would enable much more of it to be re-used in irrigation. FAO says the potential benefits of using this wastewater are enormous: a city with a population of 500,000 and a water consumption of 120 litres/day/person produces about 48,000 m3/day of wastewater. If this treated wastewater were used in irrigation, it could supply some 3,500 hectares. Nutrients in effluent are almost as important as the water itself. Typical concentrations in treated wastewater effluent from conventional sewage could provide all the nitrogen and much of the phosphorus and potassium normally required for agricultural crop production.

Drip systems generally cost in the range of $1,200-2,500 per hectare, which is too expensive for most small-scale farmers growing low-value crops. But with a few simple innovations, costs can be cut dramatically. In Cape Verde, a drip irrigation system introduced by an FAO project helped boost the island's horticulural production from 5,700 tonnes in 1991 to 17,000 tonnes in 1999. More than 20% of the irrigated area of the country has been converted to drip irrigation, and many farmers had converted from water-consuming sugar cane plantations to high-return peppers and tomatoes.

In South Asia, inexpensive treadle pumps have been successful in extracting irrigation water from shallow aquifers, allowing farmers to make good use of the household labour and increase production and income. The farmer has full control over the timing and amount of the pumped water which, given the effort involved, is used sparingly. "Treadle pumps are intrinsically pro-poor," FAO says, "as richer farmers would not be able to persuade household members to use them." Positive experience has also been reported with bucket drip-irrigation kits, which are suitable for the irrigation of small plots of vegetables and fruit trees in peri-urban areas. In Kenya, where farmers have bought more than 10 000 kits, the return on an investment of about US$15 per kit was some US$20 per month.

A range of other modern small-scale and supplementary irrigation systems could help increase productivity in rainfed areas. Pumping water with small-scale diesel or electric engines is less labour-intensive than using a treadle pump, and can be more economic than large-scale schemes that rely heavily on centralized control. Furthermore, because individual farmers are in full control of their own systems, they can often maximize production to suit their own lifestyles - something impossible with large, centrally-controlled schemes.

Drainage problems. Drainage of irrigated land serves two purposes: to reduce waterlogging and to control and reduce salinization. Proper drainage also allows crop diversification and intensification, the growth of high-yielding varieties, and effective use of inputs such as fertilizers. Drainage problems are serious on about 100-110 million hectares of irrigated land located in the world's semi-arid and arid zones. At present, about 20-30 million hectares of irrigated land are seriously damaged by the build-up of salts and 0.25-0.5 million hectares are estimated to be lost from production every year as a result of salt build-up. However, even efficient drainage causes problems - drainage effluent is often contaminated with salts, trace elements, sediments and traces of agricultural inputs, which need to be safely disposed of.

Treadle pumps from Asia to Africa

In the early 1980s, thousands of farmers in Bangladesh began using a revolutionary new device: a simple, inexpensive human-powered water pump to irrigate crops. FAO was convinced that this technology would help African farmers if it could be adapted to local conditions and manufactured locally.
In Zambia, a joint project of FAO's Special Programme for Food Security and IFAD demonstrated the benefits of the pump. Then, with assistance from the NGO International Development Enterprises, local manufacturers were trained to produce and sell the pumps.

Soon a network of retailers had spread across the country, and more than 1,000 pumps were sold at a cost of US$ 75 - 125. Instead of lugging heavy buckets of water to their small plots of beans, sweet potatoes and cassava, farmers pumped more water in less time with the treadle pump. Growers doubled their cropping area and introduced new varieties such as tomatoes, cabbage, rape and onion. Women, in particular, profited from the technology, being able to better feed their families while generating additional income.
Similar ventures with local manufacturers have started in Burkina Faso, Malawi, Mali, Senegal and United Republic of Tanzania.